In radio equipment in a satellite earth station communicating with artificial satellites, a power amplifying apparatus amplifying a signal having a microwave frequency (particularly the SHF band or higher) is needed. In recent years, in view of efficiency, maintenance, and so on, development of solid-state power amplifying apparatuses is in progress. Instead of traveling wave tube amplifiers (TWTA) using traveling wave tubes as one type of vacuum tube, solid state power amplifiers (SSPA) using field effect transistors (FET) are gaining popularity. This tendency becomes significant as development of power amplifying FETs with higher efficiency and more power advances. Particularly, FETs using material of gallium nitride (hereinafter referred to as GaN) have contributed largely to improvement in transmission output of the SSPA.
Now, the radio equipment in the satellite earth station or the like is required to comply with related regulations such as the Radio Law as well as various technical standards. A distortion characteristic expressed by third-order intermodulation product (IM3) or re-growth is one of technical standards which such radio equipment should satisfy.
As methods to improve the distortion characteristic (distortion compensation), many methods such as digital pre-distortion, feed-forward, and so on are known. As a distortion compensation method which does not require any special circuit, there has been proposed one which cancels out distortions using plural stages of FET amplifiers.
However, in satellite communication, operations with various level diagrams from high-power transmission over 100 W to low power transmission less than 1 W are expected, and thus it is required to satisfy the distortion characteristic in a wide range of transmission power.